A motion control device for executing a vehicle stabilizing control for controlling a braking force (braking torque) acting on a wheel of a vehicle, in order to maintain traveling stability of the vehicle, on the basis of a motion state quantity (yaw rate and the like) of the vehicle, has been widely known (i.e., JPH8-310366A).
Another motion control device for executing a slip restricting control for controlling the braking force (braking torque) acting on the wheel of the vehicle, by restricting a longitudinal slip of the wheel on the basis of the wheel speed of the vehicle, in order to maintain the traveling stability of the vehicle, has been also widely known.
As indicated by the graph illustrated in FIG. 14, according to a characteristic of a tire (pneumatic tire), a longitudinal force (braking force) Fx has a trade-off relationship with a lateral force Fy. The graph of FIG. 14 shows characteristics Ch1 through Ch 4 each of which indicating a Fx-Fy characteristic. Specifically, at each of the characteristics Ch1 through Ch 4, a unique wheel steer angle (e.g., slip angle α) maintained to be a constant value is set respectively, and a braking torque is increased from zero (0) in order to increase a longitudinal slip occurring at the wheel (e.g., slip ratio Sp) from 0% to 100%. The characteristic Ch 1 indicates a case where the slip angle α is relatively small, and the characteristic Ch 4 indicates a case where the slip angle α is relatively large.
For example, according to the characteristic Ch 3 in the graph of FIG. 14, in a case where the slip ratio Sp is gradually increased while the slip angle α is maintained to be a certain constant value, the relationship between Fx and Fy changes as starting from a point A corresponding to 0% of the slip ratio Sp (Sp=0%), and passing a point B, and then passing a point C at which the level of Fx reaches its maximum, and finally reaching a point D corresponding to a locked state of the tire (Sp=100%). A friction circle of the tire (wheel) indicates a range of a force generated at the tire, and a dashed line in the graph of FIG. 14 indicates the friction circle of the tire.
During the slip ratio Sp increasing process, the Fx-Fy characteristic has firstly a “range in which a decreasing amount of a lateral force Fy is small relative to an increase of a longitudinal force Fx (a braking force Fx)” (e.g., at the characteristic Ch3, a range from the point A to the vicinity of the point B), and then a “range in which the decreasing amount of the lateral force Fy is large relative to the increase of the longitudinal force Fx (the braking force Fx)” (e.g., at the characteristic Ch3, a range from the vicinity of the point B to the point D) continues.
At this point, in a case where a condition of the tire corresponds to the “range in which the decreasing amount of the lateral force Fy is small relative to the increase of the longitudinal force Fx (the braking force Fx)”, because the decreasing amount of the lateral force Fy, in other words a force being balanced with centrifugal force, is small yet, an increase of the turning radius of the vehicle may be restricted. On the other hand, in a case where the condition of the tire corresponds to the “range in which the decreasing amount of the lateral force Fy is large relative to the increase of the longitudinal force Fx (the braking force Fx)”, because the lateral force Fy has been reduced to some extent, the turning radius of the vehicle may easily increase.
The present invention has been made to deal with the above-mentioned drawbacks, and a purpose of the invention, according to the motion control device for the vehicle for maintaining the traveling stability thereof by controlling the braking force at the wheel of the vehicle, is to provide a device for restricting the Increase of the turning radius of the vehicle caused by a lack of the lateral force, in view of the lateral force at the wheel.